default for type-function is payload-type now!

Author: hannesm

documentation/source/reference.rst

@@ -377,7 +377,12 @@ language instantiates these fields.
 
    :superclasses: :class:`<field>`
 
-   :keyword type-function: A unary Dylan function computing the type.
+   :keyword type-function: A unary Dylan function computing the type of the field.
+
+   :description:
+
+      The default value for the *type-function* keyword is
+      :func:`payload-type`.
 
    See also
 

documentation/source/usage.rst

@@ -213,8 +213,6 @@ binary data definition.
       variably-typed field payload, type-function: frame.payload-type;
     end;
 
-FIXME: why is payload-type not the default type-function of a variable-typed field?
-
 The first line specifies the name ``<ethernet-frame>``, and its
 superclass, :class:`<header-frame>`.
 
@@ -235,6 +233,8 @@ binary data definition.
 
 The last field is the payload, whose type is variable and given by
 applying the function ``payload-type`` to the concrete frame instance.
+The default type-function of a :class:`<variably-typed-field>` is
+:func:`payload-type`.
 
 A payload for an ``<ethernet-frame>`` might be a ``<vlan-tag>``, if
 the ``type-code`` is ``#x8100`` (the keyword ``over`` does the hairy

fields.dylan

@@ -30,6 +30,11 @@ define function compute-static-offset (list :: <simple-vector>)
         field.fixup-function := fixup-protocol-magic;
       end;
     end;
+    if (instance?(field, <variably-typed-field>))
+      unless (slot-initialized?(field, type-function))
+        field.type-function := payload-type;
+      end;
+    end;
     if (start ~= $unknown-at-compile-time)
       unless (field.dynamic-start)
         if (field.static-start = $unknown-at-compile-time)
@@ -124,7 +129,7 @@ end;
 
 define class <variably-typed-field> (<field>)
   //type-function has to return a subclass of <container-frame>
-  slot type-function, required-init-keyword: type-function:;
+  slot type-function, init-keyword: type-function:;
 end;
 
 define abstract class <repeated-field> (<statically-typed-field>)